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Chapter 17 – Files and Streams

Chapter 17 – Files and Streams.

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Chapter 17 – Files and Streams

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  1. Chapter 17 – Files and Streams Outline17.2 Data Hierarchy17.3 Files and Streams17.4 Class File17.5 Creating a Sequential-Access File17.6 Reading Data from a Sequential-Access File17.7 Updating Sequential-Access Files17.8 Random-Access Files17.9 Creating a Random-Access File17.10 Writing Data Randomly to a Random-Access File17.11 Reading Data Sequentially from a Random-Access File17.13 New I/O APIs for the Java Platform

  2. 17.2 Data Hierarchy • Records • Composed of several related fields • Implemented as a class in Java • File is a group of related records • One field in each record is a record key • Record key is a unique identifier for a record • Sequential file • Records stored in order by record key • Database is group of related files

  3. 17.3 Files and Streams • Java views a file as a stream of bytes (Fig. 17.2) • File ends with end-of-file marker • File as a stream of bytes associated with an object • Java also associates streams with devices • System.in, System.out, and System.err • Streams can be redirected • File processing with classes in package java.io • FileInputStream • DataInputStream • FileOutputStream • DataOutputStream

  4. n-1 7 6 2 4 5 1 8 9 ... 0 3 ... end-of-file marker Fig. 17.2 Java’s view of a file of n bytes

  5. 17.3 Files and Streams • Buffering • Improves performance of I/O • Copies each output to a region of memory called a buffer • Entire buffer output to disk at once • One long disk access takes less time than many smaller ones • BufferedOutputStream buffers file output • BufferedInputStream buffers file input

  6. 17.4 Class File • Class File • Provides useful information about a file or directory • Does not open files or process files • Constructors • File (String name) can contain path information or just be file name • File (String path, String name) • File (File directory, String name) • File (URI uri) can be URL or something like file:/C:/employee.txt

  7. Fig. 17.3 File methods

  8. Import java.io package 1 // Fig. 17.4: FileTest.java 2 // Demonstrating the File class. 3 import java.awt.*; 4 import java.awt.event.*; 5 import java.io.*; 6 import javax.swing.*; 7 8 publicclass FileTest extends JFrame 9 implements ActionListener { 10 11 private JTextField enterField; 12 private JTextArea outputArea; 13 14 // set up GUI 15 public FileTest() 16 { 17 super( "Testing class File" ); 18 19 enterField = new JTextField( "Enter file or directory name here" ); 20 enterField.addActionListener( this ); 21 outputArea = new JTextArea(); 22 23 ScrollPane scrollPane = new ScrollPane(); 24 scrollPane.add( outputArea ); 25 FileTest.javaLine 5

  9. create a new File and assign it to name Body of if outputs information about the file if it exists 26 Container container = getContentPane(); 27 container.add( enterField, BorderLayout.NORTH ); 28 container.add( scrollPane, BorderLayout.CENTER ); 29 30 setSize( 400, 400 ); 31 setVisible( true ); 32 33 } // end constructor 34 35 // display information about file user specifies 36 publicvoid actionPerformed( ActionEvent actionEvent ) 37 { 38 File name = new File( actionEvent.getActionCommand() ); 39 40 // if name exists, output information about it 41 if ( name.exists() ) { 42 outputArea.setText( name.getName() + " exists\n" + 43 ( name.isFile() ? "is a file\n" : "is not a file\n" ) + 44 ( name.isDirectory() ? "is a directory\n" : 45 "is not a directory\n" ) + 46 ( name.isAbsolute() ? "is absolute path\n" : 47 "is not absolute path\n" ) + "Last modified: " + 48 name.lastModified() + "\nLength: " + name.length() + 49 "\nPath: " + name.getPath() + "\nAbsolute path: " + 50 name.getAbsolutePath() + "\nParent: " + name.getParent() ); 51 FileTest.javaLine 38Line 41

  10. Test if our object is a file Create reader to gather data from the file Read text until there is no more in the file 52 // output information if name is a file 53 if ( name.isFile() ) { 54 55 // append contents of file to outputArea 56 try { 57 BufferedReader input = new BufferedReader( 58 new FileReader( name ) ); 59 StringBuffer buffer = new StringBuffer(); 60 String text; 61 outputArea.append( "\n\n" ); 62 63 while ( ( text = input.readLine() ) != null ) 64 buffer.append( text + "\n" ); 65 66 outputArea.append( buffer.toString() ); 67 } 68 69 // process file processing problems 70 catch ( IOException ioException ) { 71 JOptionPane.showMessageDialog( this, "FILE ERROR", 72 "FILE ERROR", JOptionPane.ERROR_MESSAGE ); 73 } 74 75 } // end if 76 FileTest.javaLine 53Lines 57-58Lines 63-64

  11. Get a list of the files in the directory If file does not exist, display error 77 // output directory listing 78 elseif ( name.isDirectory() ) { 79 String directory[] = name.list(); 80 81 outputArea.append( "\n\nDirectory contents:\n"); 82 83 for ( int i = 0; i < directory.length; i++ ) 84 outputArea.append( directory[ i ] + "\n" ); 85 } 86 87 } // end outer if 88 89 // not file or directory, output error message 90 else { 91 JOptionPane.showMessageDialog( this, 92 actionEvent.getActionCommand() + " Does Not Exist", 93 "ERROR", JOptionPane.ERROR_MESSAGE ); 94 } 95 96 } // end method actionPerformed 97 98 publicstaticvoid main( String args[] ) 99 { 100 FileTest application = new FileTest(); 101 application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE ); 102 } 103 104 } // end class FileTest FileTest.javaLine 79Lines 91-93

  12. FileTest.java

  13. 17.5 Creating a Sequential-Access File • Simpler example than in book • Uses DataOutputStream instead of ObjectOutputStream • DataOutputStream is binary representation of data • Use Writer object (such as BufferedWriter) to write character stream • DataOutputStream (writes data via writeDouble, writeFloat, writeInt, writeUTF, etc.) • writeInt writes 4 byte integer • writeUTF writes 2 bytes that tell how many bytes follow

  14. 17.5 Creating a Sequential-Access File import java.io.*; ... DataOutputStream output; ... try { output = new DataOutputStream (new FileOutputStream ("accounts.txt")); } catch (IOException ioe) { System.err.println ( "File did not open properly" + ioe.toString()); System.exit (1); }

  15. 17.5 Creating a Sequential-Access File ... try { output.writeInt(int); output.writeUTF(String); output.writeFloat(float); } catch (IOException ioe) { System.err.println ( "Error writing to file" + ioe.toString()); System.exit (1); } ...

  16. 17.5 Creating a Sequential-Access File try { output.close(); } catch (IOException ioe) { System.err.println ( "File did not close properly" + ioe.toString()); System.exit (1); }

  17. 17.6 Reading Data from a Sequential-Access File • Simpler example than in book • Uses DataInputStream instead of ObjectInputStream • DataInputStream is binary representation of data • Use Reader object (such as BufferedReader) to read character stream • DataInputStream (reads data via readDouble, readFloat, readInt, readUTF (characters), etc.) • readInt reads 4 byte integer • readUTF reads 2 bytes that tell how many bytes follow • Accessing a sequential file • Data must be read in same format it was written

  18. 17.6 Reading Data from a Sequential-Access File import java.io.*; ... DataInputStream input; ... try { input = new DataInputStream (new FileInputStream ("accounts.txt")); } catch (IOException ioe) { System.err.println ( "File did not open properly" + ioe.toString()); System.exit (1); } ...

  19. 17.6 Reading Data from a Sequential-Access File try { int=input.readInt(); String=input.readUTF(); float=input.readFloat(); } catch (EOFException eof) { closeFile(); } catch (IOException ioe) { System.err.println ( "Error reading from file" + ioe.toString()); System.exit (1); } ...

  20. 17.6 Reading Data from a Sequential-Access File try { input.close(); } catch (IOException ioe) { System.err.println ( "File did not close properly" + ioe.toString()); System.exit (1); } Attempt to read past end-of-file marker throws EOFException

  21. 17.7 Updating Sequential-Access Files • Difficult to update a sequential-access file • Entire file must be rewritten to change one field • Only acceptable if many records being updated at once

  22. 17.8-17.11 Random-Access Files • “Instant-access” applications • Record must be located immediately • Transaction-processing systems require rapid access • Random-access files • Access individual records directly and quickly • Use fixed length for every record • Easy to calculate record locations • Insert records without destroying other data in file

  23. 17.8-17.11 Random-Access Files • RandomAccessFile objects have all capabilities of DataInputStream and DataOutputStream objects • data read/written at location specified by file pointer • always read data in exactly same way as it was written • seek(n), re-position file pointer to byte n of the file, n must be long integer • In code below assume that accounts.txt contains records of int account (4 bytes), String name (12 2-byte characters), float balance (4 bytes) • Each record is 32 bytes • Assume accounts 1-1000 are valid account numbers

  24. 17.8-17.11 Random-Access Files import java.io.*; ... RandomAccessFile randy; ... try { randy = new RandomAccessFile ("accounts.txt", "rw"); } catch (IOException ioe) {} ...

  25. 17.8-17.11 Random-Access Files try { if ((account >= 1) && (account <= 1000)) { randy.seek((long)(account-1)*32)); randy.writeInt(account); StringBuffer buf; buf = new StringBuffer (name); buf.setLength(12); randy.writeChars(buf.toString()); randy.writeFloat(balance); } } catch (IOException ioe) {} ...

  26. 17.8-17.11 Random-Access Files try { if ((account >= 1) && (account <= 1000)) { randy.seek((long)(account-1)*32)); account=randy.readInt(); char stuff[] = new char [12]; for (i=0; i<12; i++) { stuff[i]=randy.readChar(); } name=new String (stuff); balance=randy.readFloat(); } }

  27. 17.8-17.11 Random-Access Files catch (EOFException eof) { randy.seek(0); } catch (IOException ioe) {} ... try { randy.close(); } catch (IOException ioe) {}

  28. 17.13 New I/O APIs for the Java Platform • New in Java 1.4 (java.nio.*) • Buffers • Consolidate I/O operations • Four properties • Capacity (how much it can hold) • Limit (current end) • Position (next element to be read or written) • Mark (remembered point) • Put (write) and get (read) operations

  29. 17.13 New I/O APIs for the Java Platform • Channels • Interface Channel is a connection to an I/O device • Interacts efficiently with buffers • ReadableByteChannel interface • Method read • WriteableByteChannel interface • Method write

  30. 17.13 New I/O APIs for the Java Platform • File Locks • Restricts access to a portion of a file • FileChannel, position, size • Exclusive or shared

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